The invention applies in particular to the rapid charging of an emergency battery which is used as a source of electricity during a mains power cut. It is well known in such a case to measure the discharge time of the battery or the ampere-hours discharged and then to recharge the battery at a relatively rapid rate for a time which is a function of the previous discharge, and then to charge the battery at the maintenance charge rate or to stop charging completely as the case may be. The devices used for this purpose are relatively complicated and expensive. In some cases, a rapid fixed charge can suffice, e.g. at constant voltage, particularly when such charges are limited to occasions when the preceding discharge has exceeded a minimum duration, in order to avoid too frequent overcharging of the battery.
For this purpose it is already known to use a first timer which is automatically reset to measure the discharge time and a second timer to measure the charge time. This solution is expensive and the present invention provides a solution to the problem with a reliable and inexpensive device.